Global sea levels (also called eustatic sea level) have fluctuated over Earth’s history. These climate changes are driven by the shape and volume of ocean basins and the location of land and water.
At local scales, sea levels rise more or less than the global average due to things like groundwater pumping, natural climate variability and changes in the height of the land.
Climate change
As oceans absorb more of the heat-trapping pollution that human activities pump into the atmosphere, they are warming and rising. This is known as climate change, and it’s affecting everything from the distribution of ice sheets to the timing of peak flows in rivers and streams. The effects vary from place to place, but they all add up.
For instance, sea level rise (SLR) threatens to inundate island nations and low-lying areas around the world. Some of these areas contain vital coastal ecosystems and the world’s largest cities. Some ten percent of the global population lives within 10 meters of sea level, and it’s estimated that SLR will displace tens of millions of people by 2050.
SLR is also a key factor behind increased flood risks to cities and regions, as well as reduced freshwater availability in some places. It’s the reason why many governments are working to build sea-level rise-resistant infrastructure, including levees, dikes, walls and other engineering projects.
Since the early 1990s, when NASA and France’s space agency Centre National d’Etudes Spatiales started flying satellite altimeters, scientists have been gathering a unique space-based record of sea surface height that goes back decades. This data helps scientists track the rate at which sea levels are changing, and it’s a critical complement to tide gauges.
The SLR data has shown that the pace of change is accelerating, and the rate at which oceans are rising will likely accelerate further over the next few decades. But it’s difficult to predict how fast sea levels will rise, particularly because the underlying conditions that drive SLR are complex and variable.
Scientists are developing methods to improve our ability to predict and respond to the impacts of climate change, including SLR. They’re investigating how to integrate SLR into planning and decision-making for regional and national infrastructure projects, such as how high to build flood protections. They’re also studying the political decision-making processes that influence the design and funding of large infrastructure projects, such as bridges and dams.
It’s also important to keep in mind that even if we stopped adding carbon dioxide to the atmosphere tomorrow, it will take years, or perhaps centuries, for the Earth’s systems to respond and stabilize. And this will have impacts on communities that contributed least to the problem, like island nations and future generations.
Ocean currents
Ocean currents are continuous, directed movements of seawater that occur because of forces acting on it such as breaking waves, the Coriolis effect, temperature and salinity differences, and tides caused by the gravitational pull of the Moon and Sun. The movement of water can be horizontal, planar to the surface of the ocean, or vertical up and down within the body of water (at least 300 meters). Ocean currents travel thousands of miles and establish a global conveyer belt, transporting heat, carbon, nutrients and freshwater around the world.
For example, some currents move warmer water into the Arctic Ocean from the Gulf Stream and the Kuroshio current. This helps the Arctic ecosystem by bringing in food sources such as zooplankton. But it also increases the speed of storms and makes hurricanes stronger. And since warming ocean waters are less dense, they take up more space than cool ones, raising sea levels.
Warming oceans also affect the atmosphere above them by changing the way in which air moves, especially during storms. Warmer air can hold more moisture and may form larger clouds, which can lead to more frequent and severe rainfall. And warmer air can hold more carbon dioxide, which is known to accelerate global warming.
All of these changes make climate change a serious concern for marine life, from large fish to microscopic cyanobacteria. As the climate changes, they will have to find new places to live. And that will put them under pressure to survive, whether it’s from overfishing or pollution.
Scientists are studying how ocean currents are shifting, which will likely have a knock-on effect on the weather and climate. In particular, scientists are watching for what happens when the cold, salty, dense water of the North Atlantic sinks further into the depths because of melting ice. This could change the direction of currents, and the overall system that influences everything from people’s daily commutes to where whales migrate. NASA satellites are monitoring the ocean’s surface currents and deep currents to help understand how this complex system works. You can test your knowledge by playing Go With the Flow, an interactive ocean exploration game.
El Nino and La Nina
The overall trend of global sea levels is affected by two opposing climate patterns known as El Nino and La Nina, which occur at irregular intervals over a period of 2-7 years. These climate patterns affect global weather conditions and ocean currents, which in turn impact the ebb and flow of sea level.
An El Nino occurs when unusually warm water occurs in the equatorial Pacific, with a corresponding eastward shift of tropical precipitation. When the opposite pattern, known as La Nina, occurs, colder water forms in the equatorial Pacific with a corresponding westward shift of precipitation. These patterns are part of the larger phenomenon known as the Southern Oscillation.
During an El Nino, strong trade winds diminish or cease entirely, which reduces the upwelling of cold nutrient-rich waters off the coast of Asia. This causes the phytoplankton population to decrease and eventually trickles up the food chain to affect marine species, including fish.
El Nino episodes also contribute to wetter than normal winter and spring weather in the Deep South by redistributing moisture from the Gulf of Mexico to the region. These episodes typically produce more F2 and greater tornado outbreaks over the Deep South than would normally be expected.
During a La Nina, trade winds pick up, which results in the jet stream being placed further north. This brings colder and stormier weather to central North America, while the Deep South experiences wetter than normal weather. This weather pattern also inhibits the development of Atlantic hurricanes during the peak of the hurricane season, sparing coastal states from their associated storm damage.
Subsidence
It’s well-known that climate change is contributing to sea level rise, but less well-known that the land itself is also sinking in many coastal cities. This is a significant problem because the combination of rising seas and sinking land makes it more likely that people will be living in or near flood zones, and it’s often impossible to protect those people against flooding and other hazards.
Land subsidence is caused by a number of factors, including the natural slowing down of soil particles as they are compacted, as well as tectonic movement. However, the rate at which land sinks is also influenced by human activities such as mining and groundwater pumping. In particular, rapid population growth can lead to a rapid increase in the rate at which land is sinking, especially if it happens near river deltas and coastal areas. This is because those areas have deposits of river sediment that can be accelerated by urban development and groundwater pumping.
Scientists are able to track global land subsidence by using satellites, tide gauges, and a network of GPS sensors around the world. This allows scientists to calculate the average speed at which land is moving up and down. This data is used to create maps showing the current rates of sea level rise and land subsidence. This information is then combined with a map of the world’s coastline to show how much land is at risk of being underwater by 2100.
Sea levels are currently rising due to melting glaciers and ice sheets, as well as a thermal expansion of the ocean water. These processes will continue to occur over the course of centuries and millennia, but they are being accelerated by human activity. As a result, sea levels will likely rise for several decades at a faster rate than they would have without that acceleration.
Managing subsidence is an important part of reducing the threat to people living in coastal cities, as it can reduce the relative sea level rise by up to 20 mm per year, and this may make more of a difference in reducing the exposure of coastal populations than any other method of lowering sea levels over the next 30 years.
